As terms for representing an inter-core coupling state in a general sense, the term “coupled state” and the term “incomplete coupled state” are used. As terms for representing an inter-core coupling state in a more strict sense, the term “complete coupled state” and the term “non-coupled state” are used.
The term “coupled state” represents the coupling state in which the coupling ratio is almost 1, and the term “incomplete coupled state” represents the coupling state in which the coupling ratio is smaller than 1 but not completely 0.
The term “complete coupled state” represents the coupling state in which the coupling ratio is completely 1, and the term “non-coupled state” represents the coupling state in which the coupling ratio is so close to 0 that the coupling ratio cannot be measured.
In the field of a multicore fiber, the term “coupled multicore fiber” and the term “uncoupled multicore fiber” are used. In this case, the coupling in a “coupled multicore fiber” means that the inter-core coupling state is the “coupled state” in a general sense, and the coupling in an “uncoupled multicore fiber” means that the inter-core coupling state is the “incomplete coupled state” in a general sense.
In the present invention, the terms used in the field of a multicore fiber are used. That is, “uncoupled” in an “uncoupled multicore fiber” is not “non-coupled” in a strict sense but “incomplete coupled” in a general sense, which means a coupling state in which the coupling ratio is smaller than 1 but not completely 0.
For use in spatial multiplexed transmission using a multicore fiber, the configurations in which multiple single mode cores are accommodated in one optical fiber, such as those disclosed in Non-Patent Literature 1 and Non-Patent Literature 2, are known.
To keep individual cores in the uncoupled state, the following configurations are known: a configuration in which the cores are placed with an enough core-to-core distance, a configuration in which cores with different propagation constants are used so that the incompletely coupled state is maintained even if the cores are placed close enough, and a configuration in which a dividing layer or an air hole is provided between cores.
FIG. 24 is a diagram showing the simplest model for describing the inter-core coupling state of a multicore fiber.
An attempt to configure an uncoupled multicore fiber using homogeneous cores with the same propagation constant requires that the core-to-core interval be extended largely to avoid crosstalk between the cores, making it difficult to increase the core density. To solve this problem, an uncoupled multicore fiber uses heterogeneous cores with different propagation constants to provide a multicore fiber.
FIG. 24A shows independent waveguides of heterogeneous cores with different propagation constants β0(1) and β0(2). FIG. 24B shows an uncoupled waveguide of two types of heterogeneous cores with different propagation constants. A multicore fiber using two types of heterogeneous cores, which have different propagation constants β(1) and β(2), forms an uncoupled waveguide.
Heterogeneous cores represent cores with different propagation constants, and homogeneous cores represent cores with the same propagation constant.
Propagation constants may be made different by using different values for the parameters such as a refractive index difference, a core diameter, and a refractive index distribution. FIG. 25 shows an example of different propagation constants. FIG. 25A shows an example of the configuration of a multicore fiber composed of triangular arrangements each composed of three types of cores with different propagation constants, and FIGS. 25B to 25D show examples in which the propagation constants of the cores are made different by using different refractive index differences, core diameters, and refractive index distributions. The core shown in FIG. 25B has the core diameter of 2a1, the refractive index of n1, and the cladding refractive index of n2. The core shown in FIG. 25C has the core diameter of 2a2, the refractive index of n3, and the cladding refractive index of n2. The core shown in FIG. 25D has the core diameter of 2a3, the peak refractive index of n4, and the refractive index distribution with the cladding refractive index of n5.
The inventor of the present invention proposed a heterogeneous uncoupled multicore fiber (MCF) that suppresses inter-core coupling and accommodates cores at high density by using multiple single mode cores with different relative refractive index differences of Δ (Non Patent Literature 3). In addition, the configuration in which optical guides with different optogeometrical characteristics are used is also proposed (Patent Literature 1).
FIG. 25A shows a multicore fiber composed of multiple cores that have different propagation constants and that are arranged in triangular lattice patterns. In this example, the core-to-core distance between neighboring heterogeneous cores with different propagation constants is Λ, and the core-to-core distance between homogeneous cores with the same propagation constants is D. Note that, in a triangular lattice pattern arrangement using three types of cores, there is the geometrical relation of D=√3×Λ between the heterogeneous core-to-core distance Λ and the homogeneous core-to-core distance D.
The following describes a conventional design procedure for an uncoupled multicore fiber of heterogeneous cores with reference to FIG. 26 to FIG. 28.
For the same core-to-core distance, the crosstalk between homogeneous cores is higher than the crosstalk between heterogeneous cores and, for the same crosstalk level, the homogeneous core-to-core distance D is larger than the heterogeneous core-to-core distance Λ. Especially, when the homogeneous core-to-core distance D is determined in a triangular arrangement of three types of cores so that the crosstalk level defined for the homogeneous cores is satisfied, the crosstalk between heterogeneous cores is decreased sufficiently lower than the defined crosstalk level.
FIG. 26 is a flowchart showing the design procedure for an uncoupled multicore fiber of heterogeneous cores.
The conventional design procedure for an uncoupled multicore fiber of heterogeneous cores is as follows. First, the procedure calculates the homogeneous core-to-core distance D based on the crosstalk target value defined between the homogeneous cores (S10) and, then, calculates the heterogeneous core-to-core distance Λ from the geometrical relation of the core arrangement (S11). After that, the procedure confirms that the calculated crosstalk level for the heterogeneous core-to-core distance Λ is smaller than the crosstalk target value that is set (S12).
FIG. 27 shows the procedure for determining the homogeneous core-to-core distance D in S10. In FIG. 27A, D is the distance between the homogeneous cores with the same relative refractive index difference of Δ. The condition for design requirement is that the coupling length lc between homogeneous cores is 5000 km when the crosstalk between homogeneous cores for the propagation distance of 100 km is set equal to or lower than −30 dB.
FIG. 27B shows the relation between the coupling length lc and the homogeneous core-to-core distance D where the core diameter 2a=5 μm and the relative refractive index difference Δ is 1.10%, 1.15%, 1.20%, 1.25%, and 1.30%.
The relation shown in FIG. 27B indicates that, when the relative refractive index difference Δ is 1.20%, the homogeneous core-to-core distance D, which satisfies the condition that coupling length lc=5000 km or longer, is 40 μm.
FIG. 28 is a diagram showing crosstalk between heterogeneous cores and a core arrangement.
In FIG. 28A, the heterogeneous cores are arranged with the heterogeneous core-to-core distance Λ, determined by the homogeneous core-to-core distance D, between them. FIG. 28B shows the power conversion efficiency (also called the maximum power transfer efficiency) of heterogeneous cores. This figure shows the crosstalk for the relative refractive index different Δ2 in terms of the power conversion efficiency F when the relative refractive index different Δ1 is 1.15%, 1.20%, and 1.25%.
When the homogeneous core-to-core distance D is 40 μm, the heterogeneous core-to-core distance Λ in a triangular lattice arrangement is 23 μm (=40/√3). FIG. 28B shows the cases in which the heterogeneous core-to-core distance Λ is 10 μm, 15 μm, and 20 μm, indicating that the larger the heterogeneous core-to-core distance Λ is, the smaller the crosstalk is. If the difference in the relative refractive index differences Λ is 0.05% when the heterogeneous core-to-core distance Λ is 23 μm, the crosstalk becomes equal to or lower than −80 dB and therefore it is confirmed that the crosstalk setting value of −30 dB is satisfied.